Emerald & Amazonian Tree Boas

Aerial acrobats, these snakes often hang from a high branch with their grasping tails--and eat their meals.

Pits

A snakes pits are heat sensors, able to pick up tiny temperature differences between prey and background. Nerve signals from these pits feed into the visual system so the boa "sees" a heat image.

Snakes with heat sensors can detect, from a distance, temperature differences as small as a few thousandths of a degree. Studying snake heat sensors may help scientists design instruments to find very small tumors, which are slightly warmer than the rest of the body.

Skull

Could you swallow a 100-pound watermelon whole--without using your hands? Of course not! But many snakes do something like that every time they eat. The two halves of the lower jaw are connected by fibrous tissues that can stretch 20 to 30 times their resting length. This allows the mouth to open wide enough to consume large prey.

Teeth

A snakes teeth aren't for chewing. They're for seizing and holding prey while the boa constricts--or squeezes--its victim with its coils. Some snakes have more than 200 teeth, all replaceable.

Eye

Snakes don't have moveable eyelids. But what really makes their eyes unique is inside, at the lens. Snakes move the lens in and out to sharpen an image; most animals simply bend it. This is so unusual that scientists think snakes re-evolved sight from a nearly blind, burrowing ancestor.

Tail

Did you think snakes were ALL tail? Hardly. The tail of a snake is the part of the body behind the opening called the cloaca (clo-AY-cah) through which the snake eliminates waste. Between the head and the cloaca are the snake's organs, including what is usually a single lung.

Meet the Family

The 30-plus species in the family Boidae are medium to giant snakes, most of which produce live young. Many have thick, muscular bodies--the Green Anaconda is the world's heaviest snake--and most kill by constriction. Boas have the remnants of hind limbs, physical evidence of the group's descent from an ancestor with limbs. The earliest known true fossil boa probably dates from about 64 million years ago.

Green Anaconda (Eunectes murinus)

The most massive of all living snakes, anacondas sometimes reach 10 meters (33 feet) in length. With its eyes and nostrils on top of its head, the snake can conceal itself under water. It lives throughout the Amazon Basin.